bpo-32604: [_xxsubinterpreters] Propagate exceptions. (GH-19768)

(Note: PEP 554 is not accepted and the implementation in the code base is a private one for use in the test suite.)

If code running in a subinterpreter raises an uncaught exception then the "run" call in the calling interpreter fails. A RunFailedError is raised there that summarizes the original exception as a string. The actual exception type, __cause__, __context__, state, etc. are all discarded. This turned out to be functionally insufficient in practice. There is a more helpful solution (and PEP 554 has been updated appropriately).

This change adds the exception propagation behavior described in PEP 554 to the _xxsubinterpreters module. With this change a copy of the original exception is set to __cause__ on the RunFailedError. For now we are using "pickle", which preserves the exception's state. We also preserve the original __cause__, __context__, and __traceback__ (since "pickle" does not preserve those).

https://bugs.python.org/issue32604

1 files changed, 1018 insertions, 121 deletions

diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 8a6fce9..9c5df16 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1,5 +1,4 @@
-
-/* interpreters module */
+/* _interpreters module */
 /* low-level access to interpreter primitives */
 
 #include "Python.h"
@@ -7,35 +6,921 @@
 #include "interpreteridobject.h"
 
 
+// XXX Emit a warning?
+#define IGNORE_FAILURE(msg) \
+    fprintf(stderr, " -----\nRunFailedError: %s\n", msg); \
+    PyErr_PrintEx(0); \
+    fprintf(stderr, " -----\n"); \
+    PyErr_Clear();
+
+typedef void (*_deallocfunc)(void *);
+
+static PyInterpreterState *
+_get_current(void)
+{
+    // _PyInterpreterState_Get() aborts if lookup fails, so don't need
+    // to check the result for NULL.
+    return _PyInterpreterState_Get();
+}
+
+
+/* string utils *************************************************************/
+
+// PyMem_Free() must be used to dealocate the resulting string.
 static char *
-_copy_raw_string(PyObject *strobj)
+_strdup_and_size(const char *data, Py_ssize_t *psize, _deallocfunc *dealloc)
 {
-    const char *str = PyUnicode_AsUTF8(strobj);
-    if (str == NULL) {
-        return NULL;
+    if (data == NULL) {
+        if (psize != NULL) {
+            *psize = 0;
+        }
+        if (dealloc != NULL) {
+            *dealloc = NULL;
+        }
+        return "";
+    }
+
+    Py_ssize_t size;
+    if (psize == NULL) {
+        size = strlen(data);
+    } else {
+        size = *psize;
+        if (size == 0) {
+            size = strlen(data);
+            *psize = size;  // The size "return" value.
+        }
     }
-    char *copied = PyMem_Malloc(strlen(str)+1);
+    char *copied = PyMem_Malloc(size+1);
     if (copied == NULL) {
         PyErr_NoMemory();
         return NULL;
     }
-    strcpy(copied, str);
+    if (dealloc != NULL) {
+        *dealloc = PyMem_Free;
+    }
+    memcpy(copied, data, size+1);
     return copied;
 }
 
-static PyInterpreterState *
-_get_current(void)
+static const char *
+_pyobj_get_str_and_size(PyObject *obj, Py_ssize_t *psize)
 {
-    // PyInterpreterState_Get() aborts if lookup fails, so don't need
-    // to check the result for NULL.
-    return PyInterpreterState_Get();
+    if (PyUnicode_Check(obj)) {
+        return PyUnicode_AsUTF8AndSize(obj, psize);
+    } else {
+        const char *data = NULL;
+        PyBytes_AsStringAndSize(obj, (char **)&data, psize);
+        return data;
+    }
+}
+
+/* "raw" strings */
+
+typedef struct _rawstring {
+    Py_ssize_t size;
+    const char *data;
+    _deallocfunc dealloc;
+} _rawstring;
+
+static void
+_rawstring_init(_rawstring *raw)
+{
+    raw->size = 0;
+    raw->data = NULL;
+    raw->dealloc = NULL;
+}
+
+static _rawstring *
+_rawstring_new(void)
+{
+    _rawstring *raw = PyMem_NEW(_rawstring, 1);
+    if (raw == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    _rawstring_init(raw);
+    return raw;
+}
+
+static void
+_rawstring_clear(_rawstring *raw)
+{
+    if (raw->data != NULL && raw->dealloc != NULL) {
+        (*raw->dealloc)((void *)raw->data);
+    }
+    _rawstring_init(raw);
+}
+
+static void
+_rawstring_free(_rawstring *raw)
+{
+    _rawstring_clear(raw);
+    PyMem_Free(raw);
+}
+
+static int
+_rawstring_is_clear(_rawstring *raw)
+{
+    return raw->size == 0 && raw->data == NULL && raw->dealloc == NULL;
+}
+
+//static void
+//_rawstring_move(_rawstring *raw, _rawstring *src)
+//{
+//    raw->size = src->size;
+//    raw->data = src->data;
+//    raw->dealloc = src->dealloc;
+//    _rawstring_init(src);
+//}
+
+static void
+_rawstring_proxy(_rawstring *raw, const char *str)
+{
+    if (str == NULL) {
+        str = "";
+    }
+    raw->size = strlen(str);
+    raw->data = str;
+    raw->dealloc = NULL;
+}
+
+static int
+_rawstring_buffer(_rawstring *raw, Py_ssize_t size)
+{
+    raw->data = PyMem_Malloc(size+1);
+    if (raw->data == NULL) {
+        PyErr_NoMemory();
+        return -1;
+    }
+    raw->size = size;
+    raw->dealloc = PyMem_Free;
+    return 0;
+}
+
+static int
+_rawstring_strcpy(_rawstring *raw, const char *str, Py_ssize_t size)
+{
+    _deallocfunc dealloc = NULL;
+    const char *copied = _strdup_and_size(str, &size, &dealloc);
+    if (copied == NULL) {
+        return -1;
+    }
+
+    raw->size = size;
+    raw->dealloc = dealloc;
+    raw->data = copied;
+    return 0;
+}
+
+static int
+_rawstring_from_pyobj(_rawstring *raw, PyObject *obj)
+{
+    Py_ssize_t size = 0;
+    const char *data = _pyobj_get_str_and_size(obj, &size);
+    if (PyErr_Occurred()) {
+        return -1;
+    }
+    if (_rawstring_strcpy(raw, data, size) != 0) {
+        return -1;
+    }
+    return 0;
+}
+
+static int
+_rawstring_from_pyobj_attr(_rawstring *raw, PyObject *obj, const char *attr)
+{
+    int res = -1;
+    PyObject *valueobj = PyObject_GetAttrString(obj, attr);
+    if (valueobj == NULL) {
+        goto done;
+    }
+    if (!PyUnicode_Check(valueobj)) {
+        // XXX PyObject_Str()?  Repr()?
+        goto done;
+    }
+    const char *valuestr = PyUnicode_AsUTF8(valueobj);
+    if (valuestr == NULL) {
+        if (PyErr_Occurred()) {
+            goto done;
+        }
+    } else if (_rawstring_strcpy(raw, valuestr, 0) != 0) {
+        _rawstring_clear(raw);
+        goto done;
+    }
+    res = 0;
+
+done:
+    Py_XDECREF(valueobj);
+    return res;
+}
+
+static PyObject *
+_rawstring_as_pybytes(_rawstring *raw)
+{
+    return PyBytes_FromStringAndSize(raw->data, raw->size);
+}
+
+
+/* object utils *************************************************************/
+
+static void
+_pyobj_identify_type(PyObject *obj, _rawstring *modname, _rawstring *clsname)
+{
+    PyObject *objtype = (PyObject *)Py_TYPE(obj);
+
+    // Try __module__ and __name__.
+    if (_rawstring_from_pyobj_attr(modname, objtype, "__module__") != 0) {
+        // Fall back to the previous values in "modname".
+        IGNORE_FAILURE("bad __module__");
+    }
+    if (_rawstring_from_pyobj_attr(clsname, objtype, "__name__") != 0) {
+        // Fall back to the previous values in "clsname".
+        IGNORE_FAILURE("bad __name__");
+    }
+
+    // XXX Fall back to __qualname__?
+    // XXX Fall back to tp_name?
+}
+
+static PyObject *
+_pyobj_get_class(const char *modname, const char *clsname)
+{
+    assert(clsname != NULL);
+    if (modname == NULL) {
+        modname = "builtins";
+    }
+
+    PyObject *module = PyImport_ImportModule(modname);
+    if (module == NULL) {
+        return NULL;
+    }
+    PyObject *cls = PyObject_GetAttrString(module, clsname);
+    Py_DECREF(module);
+    return cls;
+}
+
+static PyObject *
+_pyobj_create(const char *modname, const char *clsname, PyObject *arg)
+{
+    PyObject *cls = _pyobj_get_class(modname, clsname);
+    if (cls == NULL) {
+        return NULL;
+    }
+    PyObject *obj = NULL;
+    if (arg == NULL) {
+        obj = _PyObject_CallNoArg(cls);
+    } else {
+        obj = PyObject_CallFunction(cls, "O", arg);
+    }
+    Py_DECREF(cls);
+    return obj;
+}
+
+
+/* object snapshots */
+
+typedef struct _objsnapshot {
+    // If modname is NULL then try "builtins" and "__main__".
+    _rawstring modname;
+    // clsname is required.
+    _rawstring clsname;
+
+    // The rest are optional.
+
+    // The serialized exception.
+    _rawstring *serialized;
+} _objsnapshot;
+
+static void
+_objsnapshot_init(_objsnapshot *osn)
+{
+    _rawstring_init(&osn->modname);
+    _rawstring_init(&osn->clsname);
+    osn->serialized = NULL;
+}
+
+//static _objsnapshot *
+//_objsnapshot_new(void)
+//{
+//    _objsnapshot *osn = PyMem_NEW(_objsnapshot, 1);
+//    if (osn == NULL) {
+//        PyErr_NoMemory();
+//        return NULL;
+//    }
+//    _objsnapshot_init(osn);
+//    return osn;
+//}
+
+static void
+_objsnapshot_clear(_objsnapshot *osn)
+{
+    _rawstring_clear(&osn->modname);
+    _rawstring_clear(&osn->clsname);
+    if (osn->serialized != NULL) {
+        _rawstring_free(osn->serialized);
+        osn->serialized = NULL;
+    }
+}
+
+//static void
+//_objsnapshot_free(_objsnapshot *osn)
+//{
+//    _objsnapshot_clear(osn);
+//    PyMem_Free(osn);
+//}
+
+static int
+_objsnapshot_is_clear(_objsnapshot *osn)
+{
+    return osn->serialized == NULL
+        && _rawstring_is_clear(&osn->modname)
+        && _rawstring_is_clear(&osn->clsname);
+}
+
+static void
+_objsnapshot_summarize(_objsnapshot *osn, _rawstring *rawbuf, const char *msg)
+{
+    if (msg == NULL || *msg == '\0') {
+        // XXX Keep it NULL?
+        // XXX Keep it an empty string?
+        // XXX Use something more informative?
+        msg = "<no message>";
+    }
+    const char *clsname = osn->clsname.data;
+    const char *modname = osn->modname.data;
+    if (modname && *modname == '\0') {
+        modname = NULL;
+    }
+
+    // Prep the buffer.
+    Py_ssize_t size = strlen(clsname);
+    if (modname != NULL) {
+        if (strcmp(modname, "builtins") == 0) {
+            modname = NULL;
+        } else if (strcmp(modname, "__main__") == 0) {
+            modname = NULL;
+        } else {
+            size += strlen(modname) + 1;
+        }
+    }
+    if (msg != NULL) {
+        size += strlen(": ") + strlen(msg);
+    }
+    if (modname != NULL || msg != NULL) {
+        if (_rawstring_buffer(rawbuf, size) != 0) {
+            IGNORE_FAILURE("could not summarize object snapshot");
+            return;
+        }
+    }
+    // ...else we'll proxy clsname as-is, so no need to allocate a buffer.
+
+    // XXX Use __qualname__ somehow?
+    char *buf = (char *)rawbuf->data;
+    if (modname != NULL) {
+        if (msg != NULL) {
+            snprintf(buf, size+1, "%s.%s: %s", modname, clsname, msg);
+        } else {
+            snprintf(buf, size+1, "%s.%s", modname, clsname);
+        }
+    } else if (msg != NULL) {
+        snprintf(buf, size+1, "%s: %s", clsname, msg);
+    } else {
+        _rawstring_proxy(rawbuf, clsname);
+    }
+}
+
+static _rawstring *
+_objsnapshot_get_minimal_summary(_objsnapshot *osn, PyObject *obj)
+{
+    const char *str = NULL;
+    PyObject *objstr = PyObject_Str(obj);
+    if (objstr == NULL) {
+        PyErr_Clear();
+    } else {
+        str = PyUnicode_AsUTF8(objstr);
+        if (str == NULL) {
+            PyErr_Clear();
+        }
+    }
+
+    _rawstring *summary = _rawstring_new();
+    if (summary == NULL) {
+        return NULL;
+    }
+    _objsnapshot_summarize(osn, summary, str);
+    return summary;
+}
+
+static void
+_objsnapshot_extract(_objsnapshot *osn, PyObject *obj)
+{
+    assert(_objsnapshot_is_clear(osn));
+
+    // Get the "qualname".
+    _rawstring_proxy(&osn->modname, "<unknown>");
+    _rawstring_proxy(&osn->clsname, "<unknown>");
+    _pyobj_identify_type(obj, &osn->modname, &osn->clsname);
+
+    // Serialize the object.
+    // XXX Use marshal?
+    PyObject *pickle = PyImport_ImportModule("pickle");
+    if (pickle == NULL) {
+        IGNORE_FAILURE("could not serialize object: pickle import failed");
+        return;
+    }
+    PyObject *objdata = PyObject_CallMethod(pickle, "dumps", "(O)", obj);
+    Py_DECREF(pickle);
+    if (objdata == NULL) {
+        IGNORE_FAILURE("could not serialize object: pickle.dumps failed");
+    } else {
+        _rawstring *serialized = _rawstring_new();
+        int res = _rawstring_from_pyobj(serialized, objdata);
+        Py_DECREF(objdata);
+        if (res != 0) {
+            IGNORE_FAILURE("could not serialize object: raw str failed");
+            _rawstring_free(serialized);
+        } else if (serialized->size == 0) {
+            _rawstring_free(serialized);
+        } else {
+            osn->serialized = serialized;
+        }
+    }
+}
+
+static PyObject *
+_objsnapshot_resolve_serialized(_objsnapshot *osn)
+{
+    assert(osn->serialized != NULL);
+
+    // XXX Use marshal?
+    PyObject *pickle = PyImport_ImportModule("pickle");
+    if (pickle == NULL) {
+        return NULL;
+    }
+    PyObject *objdata = _rawstring_as_pybytes(osn->serialized);
+    if (objdata == NULL) {
+        return NULL;
+    } else {
+        PyObject *obj = PyObject_CallMethod(pickle, "loads", "O", objdata);
+        Py_DECREF(objdata);
+        return obj;
+    }
+}
+
+static PyObject *
+_objsnapshot_resolve_naive(_objsnapshot *osn, PyObject *arg)
+{
+    if (_rawstring_is_clear(&osn->clsname)) {
+        // We can't proceed without at least the class name.
+        PyErr_SetString(PyExc_ValueError, "missing class name");
+        return NULL;
+    }
+
+    if (osn->modname.data != NULL) {
+        return _pyobj_create(osn->modname.data, osn->clsname.data, arg);
+    } else {
+        PyObject *obj = _pyobj_create("builtins", osn->clsname.data, arg);
+        if (obj == NULL) {
+            PyErr_Clear();
+            obj = _pyobj_create("__main__", osn->clsname.data, arg);
+        }
+        return obj;
+    }
+}
+
+static PyObject *
+_objsnapshot_resolve(_objsnapshot *osn)
+{
+    if (osn->serialized != NULL) {
+        PyObject *obj = _objsnapshot_resolve_serialized(osn);
+        if (obj != NULL) {
+            return obj;
+        }
+        IGNORE_FAILURE("could not de-serialize object");
+    }
+
+    // Fall back to naive resolution.
+    return _objsnapshot_resolve_naive(osn, NULL);
+}
+
+
+/* exception utils **********************************************************/
+
+// _pyexc_create is inspired by _PyErr_SetObject().
+
+static PyObject *
+_pyexc_create(PyObject *exctype, const char *msg, PyObject *tb)
+{
+    assert(exctype != NULL && PyExceptionClass_Check(exctype));
+
+    PyObject *curtype = NULL, *curexc = NULL, *curtb = NULL;
+    PyErr_Fetch(&curtype, &curexc, &curtb);
+
+    // Create the object.
+    PyObject *exc = NULL;
+    if (msg != NULL) {
+        PyObject *msgobj = PyUnicode_FromString(msg);
+        if (msgobj == NULL) {
+            IGNORE_FAILURE("could not deserialize propagated error message");
+        }
+        exc = _PyObject_CallOneArg(exctype, msgobj);
+        Py_XDECREF(msgobj);
+    } else {
+        exc = _PyObject_CallNoArg(exctype);
+    }
+    if (exc == NULL) {
+        return NULL;
+    }
+
+    // Set the traceback, if any.
+    if (tb == NULL) {
+        tb = curtb;
+    }
+    if (tb != NULL) {
+        // This does *not* steal a reference!
+        PyException_SetTraceback(exc, tb);
+    }
+
+    PyErr_Restore(curtype, curexc, curtb);
+
+    return exc;
+}
+
+/* traceback snapshots */
+
+typedef struct _tbsnapshot {
+    _rawstring tbs_funcname;
+    _rawstring tbs_filename;
+    int tbs_lineno;
+    struct _tbsnapshot *tbs_next;
+} _tbsnapshot;
+
+static void
+_tbsnapshot_init(_tbsnapshot *tbs)
+{
+    _rawstring_init(&tbs->tbs_funcname);
+    _rawstring_init(&tbs->tbs_filename);
+    tbs->tbs_lineno = -1;
+    tbs->tbs_next = NULL;
+}
+
+static _tbsnapshot *
+_tbsnapshot_new(void)
+{
+    _tbsnapshot *tbs = PyMem_NEW(_tbsnapshot, 1);
+    if (tbs == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    _tbsnapshot_init(tbs);
+    return tbs;
+}
+
+static void _tbsnapshot_free(_tbsnapshot *);  // forward
+
+static void
+_tbsnapshot_clear(_tbsnapshot *tbs)
+{
+    _rawstring_clear(&tbs->tbs_funcname);
+    _rawstring_clear(&tbs->tbs_filename);
+    tbs->tbs_lineno = -1;
+    if (tbs->tbs_next != NULL) {
+        _tbsnapshot_free(tbs->tbs_next);
+        tbs->tbs_next = NULL;
+    }
+}
+
+static void
+_tbsnapshot_free(_tbsnapshot *tbs)
+{
+    _tbsnapshot_clear(tbs);
+    PyMem_Free(tbs);
+}
+
+static int
+_tbsnapshot_is_clear(_tbsnapshot *tbs)
+{
+    return tbs->tbs_lineno == -1 && tbs->tbs_next == NULL
+        && _rawstring_is_clear(&tbs->tbs_funcname)
+        && _rawstring_is_clear(&tbs->tbs_filename);
+}
+
+static int
+_tbsnapshot_from_pytb(_tbsnapshot *tbs, PyTracebackObject *pytb)
+{
+    assert(_tbsnapshot_is_clear(tbs));
+    assert(pytb != NULL);
+
+    PyCodeObject *pycode = pytb->tb_frame->f_code;
+    const char *funcname = PyUnicode_AsUTF8(pycode->co_name);
+    if (_rawstring_strcpy(&tbs->tbs_funcname, funcname, 0) != 0) {
+        goto error;
+    }
+    const char *filename = PyUnicode_AsUTF8(pycode->co_filename);
+    if (_rawstring_strcpy(&tbs->tbs_filename, filename, 0) != 0) {
+        goto error;
+    }
+    tbs->tbs_lineno = pytb->tb_lineno;
+
+    return 0;
+
+error:
+    _tbsnapshot_clear(tbs);
+    return -1;
+}
+
+static int
+_tbsnapshot_extract(_tbsnapshot *tbs, PyTracebackObject *pytb)
+{
+    assert(_tbsnapshot_is_clear(tbs));
+    assert(pytb != NULL);
+
+    _tbsnapshot *next = NULL;
+    while (pytb->tb_next != NULL) {
+        _tbsnapshot *_next = _tbsnapshot_new();
+        if (_next == NULL) {
+            goto error;
+        }
+        if (_tbsnapshot_from_pytb(_next, pytb) != 0) {
+            goto error;
+        }
+        if (next != NULL) {
+            _next->tbs_next = next;
+        }
+        next = _next;
+        pytb = pytb->tb_next;
+    }
+    if (_tbsnapshot_from_pytb(tbs, pytb) != 0) {
+        goto error;
+    }
+    tbs->tbs_next = next;
+
+    return 0;
+
+error:
+    _tbsnapshot_clear(tbs);
+    return -1;
+}
+
+static PyObject *
+_tbsnapshot_resolve(_tbsnapshot *tbs)
+{
+    assert(!PyErr_Occurred());
+    // At this point there should be no traceback set yet.
+
+    while (tbs != NULL) {
+        const char *funcname = tbs->tbs_funcname.data;
+        const char *filename = tbs->tbs_filename.data;
+        _PyTraceback_Add(funcname ? funcname : "",
+                         filename ? filename : "",
+                         tbs->tbs_lineno);
+        tbs = tbs->tbs_next;
+    }
+
+    PyObject *exctype = NULL, *excval = NULL, *tb = NULL;
+    PyErr_Fetch(&exctype, &excval, &tb);
+    // Leave it cleared.
+    return tb;
+}
+
+/* exception snapshots */
+
+typedef struct _excsnapshot {
+    _objsnapshot es_object;
+    _rawstring *es_msg;
+    struct _excsnapshot *es_cause;
+    struct _excsnapshot *es_context;
+    char es_suppress_context;
+    struct _tbsnapshot *es_traceback;
+} _excsnapshot;
+
+static void
+_excsnapshot_init(_excsnapshot *es)
+{
+    _objsnapshot_init(&es->es_object);
+    es->es_msg = NULL;
+    es->es_cause = NULL;
+    es->es_context = NULL;
+    es->es_suppress_context = 0;
+    es->es_traceback = NULL;
+}
+
+static _excsnapshot *
+_excsnapshot_new(void) {
+    _excsnapshot *es = PyMem_NEW(_excsnapshot, 1);
+    if (es == NULL) {
+        PyErr_NoMemory();
+        return NULL;
+    }
+    _excsnapshot_init(es);
+    return es;
+}
+
+static void _excsnapshot_free(_excsnapshot *);  // forward
+
+static void
+_excsnapshot_clear(_excsnapshot *es)
+{
+    _objsnapshot_clear(&es->es_object);
+    if (es->es_msg != NULL) {
+        _rawstring_free(es->es_msg);
+        es->es_msg = NULL;
+    }
+    if (es->es_cause != NULL) {
+        _excsnapshot_free(es->es_cause);
+        es->es_cause = NULL;
+    }
+    if (es->es_context != NULL) {
+        _excsnapshot_free(es->es_context);
+        es->es_context = NULL;
+    }
+    es->es_suppress_context = 0;
+    if (es->es_traceback != NULL) {
+        _tbsnapshot_free(es->es_traceback);
+        es->es_traceback = NULL;
+    }
+}
+
+static void
+_excsnapshot_free(_excsnapshot *es)
+{
+    _excsnapshot_clear(es);
+    PyMem_Free(es);
+}
+
+static int
+_excsnapshot_is_clear(_excsnapshot *es)
+{
+    return es->es_suppress_context == 0
+        && es->es_cause == NULL
+        && es->es_context == NULL
+        && es->es_traceback == NULL
+        && es->es_msg == NULL
+        && _objsnapshot_is_clear(&es->es_object);
+}
+
+static PyObject *
+_excsnapshot_get_exc_naive(_excsnapshot *es)
+{
+    _rawstring buf;
+    const char *msg = NULL;
+    if (es->es_msg != NULL) {
+        msg = es->es_msg->data;
+    } else {
+        _objsnapshot_summarize(&es->es_object, &buf, NULL);
+        if (buf.size > 0) {
+            msg = buf.data;
+        }
+    }
+
+    PyObject *exc = NULL;
+    // XXX Use _objsnapshot_resolve_naive()?
+    const char *modname = es->es_object.modname.size > 0
+        ? es->es_object.modname.data
+        : NULL;
+    PyObject *exctype = _pyobj_get_class(modname, es->es_object.clsname.data);
+    if (exctype != NULL) {
+        exc = _pyexc_create(exctype, msg, NULL);
+        Py_DECREF(exctype);
+        if (exc != NULL) {
+            return exc;
+        }
+        PyErr_Clear();
+    } else {
+        PyErr_Clear();
+    }
+    exctype = PyExc_Exception;
+    return _pyexc_create(exctype, msg, NULL);
+}
+
+static PyObject *
+_excsnapshot_get_exc(_excsnapshot *es)
+{
+    assert(!_objsnapshot_is_clear(&es->es_object));
+
+    PyObject *exc = _objsnapshot_resolve(&es->es_object);
+    if (exc == NULL) {
+        // Fall back to resolving the object.
+        PyObject *curtype = NULL, *curexc = NULL, *curtb = NULL;
+        PyErr_Fetch(&curtype, &curexc, &curtb);
+
+        exc = _excsnapshot_get_exc_naive(es);
+        if (exc == NULL) {
+            PyErr_Restore(curtype, curexc, curtb);
+            return NULL;
+        }
+    }
+    // People can do some weird stuff...
+    if (!PyExceptionInstance_Check(exc)) {
+        // We got a bogus "exception".
+        Py_DECREF(exc);
+        PyErr_SetString(PyExc_TypeError, "expected exception");
+        return NULL;
+    }
+    return exc;
+}
+
+static void _excsnapshot_extract(_excsnapshot *, PyObject *);
+static void
+_excsnapshot_extract(_excsnapshot *es, PyObject *excobj)
+{
+    assert(_excsnapshot_is_clear(es));
+    assert(PyExceptionInstance_Check(excobj));
+
+    _objsnapshot_extract(&es->es_object, excobj);
+
+    es->es_msg = _objsnapshot_get_minimal_summary(&es->es_object, excobj);
+    if (es->es_msg == NULL) {
+        PyErr_Clear();
+    }
+
+    PyBaseExceptionObject *exc = (PyBaseExceptionObject *)excobj;
+
+    if (exc->cause != NULL && exc->cause != Py_None) {
+        es->es_cause = _excsnapshot_new();
+        _excsnapshot_extract(es->es_cause, exc->cause);
+    }
+
+    if (exc->context != NULL && exc->context != Py_None) {
+        es->es_context = _excsnapshot_new();
+        _excsnapshot_extract(es->es_context, exc->context);
+    }
+
+    es->es_suppress_context = exc->suppress_context;
+
+    PyObject *tb = PyException_GetTraceback(excobj);
+    if (PyErr_Occurred()) {
+        IGNORE_FAILURE("could not get traceback");
+    } else if (tb == Py_None) {
+        Py_DECREF(tb);
+        tb = NULL;
+    }
+    if (tb != NULL) {
+        es->es_traceback = _tbsnapshot_new();
+        if (_tbsnapshot_extract(es->es_traceback,
+                                (PyTracebackObject *)tb) != 0) {
+            IGNORE_FAILURE("could not extract __traceback__");
+        }
+    }
+}
+
+static PyObject *
+_excsnapshot_resolve(_excsnapshot *es)
+{
+    PyObject *exc = _excsnapshot_get_exc(es);
+    if (exc == NULL) {
+        return NULL;
+    }
+
+    if (es->es_traceback != NULL) {
+        PyObject *tb = _tbsnapshot_resolve(es->es_traceback);
+        if (tb == NULL) {
+            // The snapshot is still somewhat useful without this.
+            IGNORE_FAILURE("could not deserialize traceback");
+        } else {
+            // This does not steal references.
+            PyException_SetTraceback(exc, tb);
+            Py_DECREF(tb);
+        }
+    }
+    // NULL means "not set".
+
+    if (es->es_context != NULL) {
+        PyObject *context = _excsnapshot_resolve(es->es_context);
+        if (context == NULL) {
+            // The snapshot is still useful without this.
+            IGNORE_FAILURE("could not deserialize __context__");
+        } else {
+            // This steals references but we have one to give.
+            PyException_SetContext(exc, context);
+        }
+    }
+    // NULL means "not set".
+
+    if (es->es_cause != NULL) {
+        PyObject *cause = _excsnapshot_resolve(es->es_cause);
+        if (cause == NULL) {
+            // The snapshot is still useful without this.
+            IGNORE_FAILURE("could not deserialize __cause__");
+        } else {
+            // This steals references, but we have one to give.
+            PyException_SetCause(exc, cause);
+        }
+    }
+    // NULL means "not set".
+
+    ((PyBaseExceptionObject *)exc)->suppress_context = es->es_suppress_context;
+
+    return exc;
 }
 
 
 /* data-sharing-specific code ***********************************************/
 
+/* shared "object" */
+
 struct _sharednsitem {
-    char *name;
+    _rawstring name;
     _PyCrossInterpreterData data;
 };
 
@@ -44,8 +929,7 @@ static void _sharednsitem_clear(struct _sharednsitem *);  // forward
 static int
 _sharednsitem_init(struct _sharednsitem *item, PyObject *key, PyObject *value)
 {
-    item->name = _copy_raw_string(key);
-    if (item->name == NULL) {
+    if (_rawstring_from_pyobj(&item->name, key) != 0) {
         return -1;
     }
     if (_PyObject_GetCrossInterpreterData(value, &item->data) != 0) {
@@ -58,17 +942,14 @@ _sharednsitem_init(struct _sharednsitem *item, PyObject *key, PyObject *value)
 static void
 _sharednsitem_clear(struct _sharednsitem *item)
 {
-    if (item->name != NULL) {
-        PyMem_Free(item->name);
-        item->name = NULL;
-    }
+    _rawstring_clear(&item->name);
     _PyCrossInterpreterData_Release(&item->data);
 }
 
 static int
 _sharednsitem_apply(struct _sharednsitem *item, PyObject *ns)
 {
-    PyObject *name = PyUnicode_FromString(item->name);
+    PyObject *name = PyUnicode_FromString(item->name.data);
     if (name == NULL) {
         return -1;
     }
@@ -159,121 +1040,119 @@ _sharedns_apply(_sharedns *shared, PyObject *ns)
     return 0;
 }
 
+/* shared exception */
+
 // Ultimately we'd like to preserve enough information about the
 // exception and traceback that we could re-constitute (or at least
 // simulate, a la traceback.TracebackException), and even chain, a copy
 // of the exception in the calling interpreter.
 
 typedef struct _sharedexception {
-    char *name;
-    char *msg;
+    _excsnapshot snapshot;
+    _rawstring msg;
 } _sharedexception;
 
+static void
+_sharedexception_init(_sharedexception *she)
+{
+    _excsnapshot_init(&she->snapshot);
+    _rawstring_init(&she->msg);
+}
+
 static _sharedexception *
 _sharedexception_new(void)
 {
-    _sharedexception *err = PyMem_NEW(_sharedexception, 1);
-    if (err == NULL) {
+    _sharedexception *she = PyMem_NEW(_sharedexception, 1);
+    if (she == NULL) {
         PyErr_NoMemory();
         return NULL;
     }
-    err->name = NULL;
-    err->msg = NULL;
-    return err;
+    _sharedexception_init(she);
+    return she;
 }
 
 static void
-_sharedexception_clear(_sharedexception *exc)
+_sharedexception_clear(_sharedexception *she)
 {
-    if (exc->name != NULL) {
-        PyMem_Free(exc->name);
-    }
-    if (exc->msg != NULL) {
-        PyMem_Free(exc->msg);
-    }
+    _excsnapshot_clear(&she->snapshot);
+    _rawstring_clear(&she->msg);
 }
 
 static void
-_sharedexception_free(_sharedexception *exc)
+_sharedexception_free(_sharedexception *she)
 {
-    _sharedexception_clear(exc);
-    PyMem_Free(exc);
+    _sharedexception_clear(she);
+    PyMem_Free(she);
 }
 
-static _sharedexception *
-_sharedexception_bind(PyObject *exctype, PyObject *exc, PyObject *tb)
+static int
+_sharedexception_is_clear(_sharedexception *she)
 {
-    assert(exctype != NULL);
-    char *failure = NULL;
-
-    _sharedexception *err = _sharedexception_new();
-    if (err == NULL) {
-        goto finally;
-    }
+    return 1
+        && _excsnapshot_is_clear(&she->snapshot)
+        && _rawstring_is_clear(&she->msg);
+}
 
-    PyObject *name = PyUnicode_FromFormat("%S", exctype);
-    if (name == NULL) {
-        failure = "unable to format exception type name";
-        goto finally;
-    }
-    err->name = _copy_raw_string(name);
-    Py_DECREF(name);
-    if (err->name == NULL) {
-        if (PyErr_ExceptionMatches(PyExc_MemoryError)) {
-            failure = "out of memory copying exception type name";
-        } else {
-            failure = "unable to encode and copy exception type name";
+static PyObject *
+_sharedexception_get_cause(_sharedexception *sharedexc)
+{
+    // FYI, "cause" is already normalized.
+    PyObject *cause = _excsnapshot_resolve(&sharedexc->snapshot);
+    if (cause == NULL) {
+        if (PyErr_Occurred()) {
+            IGNORE_FAILURE("could not deserialize exc snapshot");
         }
-        goto finally;
+        return NULL;
     }
+    // XXX Ensure "cause" has a traceback.
+    return cause;
+}
 
-    if (exc != NULL) {
-        PyObject *msg = PyUnicode_FromFormat("%S", exc);
-        if (msg == NULL) {
-            failure = "unable to format exception message";
-            goto finally;
-        }
-        err->msg = _copy_raw_string(msg);
-        Py_DECREF(msg);
-        if (err->msg == NULL) {
-            if (PyErr_ExceptionMatches(PyExc_MemoryError)) {
-                failure = "out of memory copying exception message";
-            } else {
-                failure = "unable to encode and copy exception message";
-            }
-            goto finally;
-        }
-    }
+static void
+_sharedexception_extract(_sharedexception *she, PyObject *exc)
+{
+    assert(_sharedexception_is_clear(she));
+    assert(exc != NULL);
 
-finally:
-    if (failure != NULL) {
-        PyErr_Clear();
-        if (err->name != NULL) {
-            PyMem_Free(err->name);
-            err->name = NULL;
+    _excsnapshot_extract(&she->snapshot, exc);
+
+    // Compose the message.
+    const char *msg = NULL;
+    PyObject *msgobj = PyUnicode_FromFormat("%S", exc);
+    if (msgobj == NULL) {
+        IGNORE_FAILURE("unable to format exception message");
+    } else {
+        msg = PyUnicode_AsUTF8(msgobj);
+        if (PyErr_Occurred()) {
+            PyErr_Clear();
         }
-        err->msg = failure;
     }
-    return err;
+    _objsnapshot_summarize(&she->snapshot.es_object, &she->msg, msg);
+    Py_XDECREF(msgobj);
 }
 
-static void
-_sharedexception_apply(_sharedexception *exc, PyObject *wrapperclass)
+static PyObject *
+_sharedexception_resolve(_sharedexception *sharedexc, PyObject *wrapperclass)
 {
-    if (exc->name != NULL) {
-        if (exc->msg != NULL) {
-            PyErr_Format(wrapperclass, "%s: %s",  exc->name, exc->msg);
-        }
-        else {
-            PyErr_SetString(wrapperclass, exc->name);
-        }
-    }
-    else if (exc->msg != NULL) {
-        PyErr_SetString(wrapperclass, exc->msg);
-    }
-    else {
-        PyErr_SetNone(wrapperclass);
+    assert(!PyErr_Occurred());
+
+    // Get the exception object (already normalized).
+    PyObject *exc = _pyexc_create(wrapperclass, sharedexc->msg.data, NULL);
+    assert(exc != NULL);
+
+    // Set __cause__, is possible.
+    PyObject *cause = _sharedexception_get_cause(sharedexc);
+    if (cause != NULL) {
+        // Set __context__.
+        Py_INCREF(cause);  // PyException_SetContext() steals a reference.
+        PyException_SetContext(exc, cause);
+
+        // Set __cause__.
+        Py_INCREF(cause);  // PyException_SetCause() steals a reference.
+        PyException_SetCause(exc, cause);
     }
+
+    return exc;
 }
 
 
@@ -1869,11 +2748,9 @@ _ensure_not_running(PyInterpreterState *interp)
 
 static int
 _run_script(PyInterpreterState *interp, const char *codestr,
-            _sharedns *shared, _sharedexception **exc)
+            _sharedns *shared, _sharedexception **pexc)
 {
-    PyObject *exctype = NULL;
-    PyObject *excval = NULL;
-    PyObject *tb = NULL;
+    assert(!PyErr_Occurred());  // ...in the called interpreter.
 
     PyObject *main_mod = _PyInterpreterState_GetMainModule(interp);
     if (main_mod == NULL) {
@@ -1904,25 +2781,38 @@ _run_script(PyInterpreterState *interp, const char *codestr,
         Py_DECREF(result);  // We throw away the result.
     }
 
-    *exc = NULL;
+    *pexc = NULL;
     return 0;
 
+    PyObject *exctype = NULL, *exc = NULL, *tb = NULL;
 error:
-    PyErr_Fetch(&exctype, &excval, &tb);
+    PyErr_Fetch(&exctype, &exc, &tb);
 
-    _sharedexception *sharedexc = _sharedexception_bind(exctype, excval, tb);
-    Py_XDECREF(exctype);
-    Py_XDECREF(excval);
-    Py_XDECREF(tb);
-    if (sharedexc == NULL) {
-        fprintf(stderr, "RunFailedError: script raised an uncaught exception");
-        PyErr_Clear();
-        sharedexc = NULL;
+    // First normalize the exception.
+    PyErr_NormalizeException(&exctype, &exc, &tb);
+    assert(PyExceptionInstance_Check(exc));
+    if (tb != NULL) {
+        PyException_SetTraceback(exc, tb);
     }
-    else {
+
+    // Behave as though the exception was caught in this thread.
+    PyErr_SetExcInfo(exctype, exc, tb);  // Like entering "except" block.
+
+    // Serialize the exception.
+    _sharedexception *sharedexc = _sharedexception_new();
+    if (sharedexc == NULL) {
+        IGNORE_FAILURE("script raised an uncaught exception");
+    } else {
+        _sharedexception_extract(sharedexc, exc);
         assert(!PyErr_Occurred());
     }
-    *exc = sharedexc;
+
+    // Clear the exception.
+    PyErr_SetExcInfo(NULL, NULL, NULL);  // Like leaving "except" block.
+    PyErr_Clear();  // Do not re-raise.
+
+    // "Return" the serialized exception.
+    *pexc = sharedexc;
     return -1;
 }
 
@@ -1930,6 +2820,8 @@ static int
 _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
                            PyObject *shareables)
 {
+    assert(!PyErr_Occurred());  // ...in the calling interpreter.
+
     if (_ensure_not_running(interp) < 0) {
         return -1;
     }
@@ -1963,8 +2855,8 @@ _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
     }
 
     // Run the script.
-    _sharedexception *exc = NULL;
-    int result = _run_script(interp, codestr, shared, &exc);
+    _sharedexception *sharedexc = NULL;
+    int result = _run_script(interp, codestr, shared, &sharedexc);
 
     // Switch back.
     if (save_tstate != NULL) {
@@ -1973,9 +2865,14 @@ _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
 #endif
 
     // Propagate any exception out to the caller.
-    if (exc != NULL) {
-        _sharedexception_apply(exc, RunFailedError);
-        _sharedexception_free(exc);
+    if (sharedexc != NULL) {
+        assert(!PyErr_Occurred());
+        PyObject *exc = _sharedexception_resolve(sharedexc, RunFailedError);
+        // XXX This is not safe once interpreters no longer share allocators.
+        _sharedexception_free(sharedexc);
+        PyObject *exctype = (PyObject *)Py_TYPE(exc);
+        Py_INCREF(exctype);  // PyErr_Restore() steals a reference.
+        PyErr_Restore(exctype, exc, PyException_GetTraceback(exc));
     }
     else if (result != 0) {
         // We were unable to allocate a shared exception.